Question

Why does the night-sky in spite of the presence of countless stars appear dark?
Asked by:
Raza

Answer

Many scientists in the past have argued - just like you - that if the universe is infinite, then anywhere we look we would see a star. Therefore the entire sky should be - not just bright - but intensely hot too! The problem became known as Olbers' paradox because it was formulated explicitly by the Viennese astronomer, Heinrich Olbers, in 1826.

At first, it was suggested that light rays from distant stars is absorbed on its way to us by intergalactic matter. However, astronomers soon realised that this was no solution: any matter drifting out in space that was coming between us and all this starlight would simply heat up so much that it would begin to emit light itself.

Olbers' paradox rests on a number of assumptions: that the universe is of infinite size; that it is infinitely old; and that it is not expanding. Each of these assumptions may actually turn out to be false in the universe we live in and therefore provide a solution to the puzzle.

It is presently believed, for example, that the universe hasn't existed in the form we know it for ever, but rather that it is approximately 14 billion years old. This gives us a loophole to escape the paradox, because even if the universe is infinite and there are infinite stars out there, we would only see those whose light has had enough time to reach us since the beginning of time.

There is also much evidence to suggest that the universe is expanding, causing beams of light which travel through spacetime to be stretched to longer wavelengths. By the time they reach the Earth, many photons have wavelengths that are out of the range of visible light or are even too large to be detected. Calculations using the present estimates for the age and expansion of the universe suggest that this is less important in reducing the brightness of the night sky than the age-factor.

Finally, it may also turn out that the universe does not contain an infinite amount of matter. If it is finite in size, and there are only a finite number of stars in the sky, then it is plausible that there just aren't enough stars out there to fill up the night sky.
Answered by:
Sally Riordan, M.A., Management Consultant, London, UK

Science Quote

'The difference between what the most and the least learned people know is inexpressibly trivial in relation to that which is unknown.'